A digital microwave communication system has a merit compared with a wired circuit such as an optical fiber that it is inexpensive and construction in a short period is possible. For this reason, in recent years, the digital microwave communication system is popularly employed as a backhaul circuit in a mobile network. And by development of mobile broadband communication represented by an LTE (Long Term Evolution) and a mobile WiMAX (Worldwide Interoperability for Microwave Access), further capacity increase is required for the backhaul circuit.
Generally, for the digital microwave communication system, circuit design is performed according to the standard specified by such as G.826 standardized by ITU-T (International Telecommunication Union—Telecommunication Standardization Sector). Therefore, the digital microwave communication system is operated in most cases throughout a year in a state that has enough margins for circuit quality. In recent years, in order to improve utilization efficiencies and availabilities of the circuit further, an adaptive modulation system which varies a modulation system according to a state of the propagation line adaptively is becoming to be used (for example, refer to patent document 1).
In an adaptive modulation system disclosed in patent document 1, at receiving side, an appropriate modulation system is selected using such as received signal level and error rate information of a demodulated signal, and the selected result is transmitted to an opposite station side. On the opposite station side, a transmission modulation system is switched following the selected result. For example, in case propagation line state of a radio wave is good, a modulation system with higher transmission rate is applied. Accordingly, compared with a system of a conventional fixed modulation system, communication capacity can be increased. Conversely, in case propagation line state is deteriorated, a modulation system which is applicable to the environment, for example, a modulation system with a small modulation multi-valued number is applied. Accordingly, because circuit disconnection can be prevented, circuit operating ratio can be improved.
Adaptive modulation control which controls a modulation system and transmission power of an opposite station also exists (for example, refer to patent document 2). In an adaptive modulation system disclosed in patent document 2, change in the modulation system and the transmission power is requested to the opposite side device based on a difference between a current reception level and a required reception level set in advance.